A number of reproduction methods are only capable of reproducing a small number of image tones. For example, offset printing or electrophotographic printing methods are only capable of printing two tone values i.e. deposit ink or toner or not. In order to reproduce images having continuous tones, a halftoning or screening technique is used.
A halftoning technique converts a density value into a geometric distribution of binary dots that can be printed. The eye is not capable of seeing the individual halftone dots, and only sees the corresponding "spatially integrated" density value.
For digital halftoning high resolution laser recorders are used to generate halftone dots on an imaging element, comprising a photographic film, paper or plate. The laser beam scans the film or plate on a line-by-line basis. Within every line the laser beam can be modulated "on" or "off" at discrete positions. In this way, an addressable "grid" is formed of lines and columns.
Two main classes of halftoning techniques have been described for use in the graphic arts field. These two techniques are known as "amplitude modulation screening or autotypical screening" (abbreviated as AM) and "frequency modulation screening or stochastic screening" (abbreviated as FM). Reference is made to FIGS. 1 and 2, wherein FIG. 1 illustrates an arrangement as used in amplitude modulation and comprising microdots 2 clustered together into a halftone dot 1; and wherein FIG. 2 illustrates an arrangement of microdots 2 as used in frequency modulation. For a clear understanding, most of the relevant technical terms used in the present application are explained in a separate chapter at the beginning of the detailed description (see later on).
According to amplitude modulation screening, the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots the different tones of an image can be simulated. Consequently this AM-technique can also be called "dot size modulation screening". FIG. 5 shows, by means of an example according to prior art, how increasing the size of halftone dots in a halftone pattern gives a denser image. FIG. 8.1 shows an exemplary AM-halftone dot, whereas FIG. 8.2 shows a bitmap configuration of said halftone dot, and whereas FIG. 8.3 shows a representation of said bitmap when saved in an electronic memory.
Said AM-halftone technique is often used in combination with a digital film recorder, which consists of a scanning laser beam exposing a photosensitive material at high resolution. The photosensitive material, generally called "imaging element", can be a photographic film, from which later on a printing plate is prepared by means of photomechanical techniques.
As the present invention relates more specifically to a method for preparing a lithographic printing plate, in particular to a method for preparing a lithographic printing plate comprising the steps of informationwise exposing an imaging element and thereafter processing the exposed imaging element by a diffusion transfer process, some additional background is given hereinafter.
Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (called "oleophilic" areas) whereas other areas will not accept ink (called "oleophobic" areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.
Two basic types of lithographic printing plates are known. According to a first type, so-called "wet" printing plates, either water or an aqueous dampening liquid and ink are applied to the plate surface that includes hydrophilic and hydrophobic areas. The hydrophilic areas are soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink. A second type of lithographic printing plate operates without the use of a dampening liquid and is called "driographic" printing plate. This type of printing plate comprises highly ink repellant areas and oleophilic areas.
Lithographic printing plates can be prepared using a photosensitive lithographic printing plate precursor, referred to herein as an "imaging element". Such an imaging element is exposed in accordance with the image data and is generally developed thereafter so that a differentiation results in ink accepting properties between the exposed and unexposed areas.
Silver salt diffusion transfer processes are known and have been described, for example, in U.S. Pat. No. 2,352,042 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York (1972).
From the above it will be clear that lithographic printing is only capable of reproducing two tone values because the areas will either accept ink or not. Thus lithographic printing is a so-called "binary" process. As mentioned hereabove, in order to reproduce originals having continuously changing tone values by such processes, halftone screening techniques are applied. Yet the rendering of small dots still presents an important problem as is explained hereinafter.
Laser imagesetters and "direct to plate recorders or platesetters" expose halftone images on graphic arts film and plates by means of laser beam scanning and modulation. The faithful rendition of halftone levels, represented by binary bitmap images, is difficult to achieve because the image is distorted by the gaussian intensity distribution of the laser beam (FIG. 3 sketches a three-dimensional distribution of a Gaussian laser beam) and by the sensitometric characteristics of the film and plate material. This distortion changes the rendition of the halftone levels: small dots (positive or negative) in lowtones or highlights and small dots in hightones or shadows may be rendered too small (over- and underexposure) and halftone dots tend to print unevenly or not at all. Generally, a black dot in a white area is called "a highlight", whereas a white dot in a dark area is called "a shadow".
This distortion will thus be most noticeable in small dots, wherein not only the edges but also the density of the dot will not be rendered optimally. At the other end of the tone scale small unexposed areas, will become fogged by the influence of light beams exposing the surrounding area. This means that the faithful rendition of small dots or small holes can be extremely difficult.
As to a possible solution for this problem, some different approaches might be undertaken.
First, an overall compensation by applying a higher or lower laser power is not acceptable because overexposure makes the small highlight dots larger but fills up the small shadow dots. Underexposure reverses this effect, opening up the shadows but reducing the highlights. Both over- and underexposure will reduce the number of rendered halftone levels, and hence also the tonal range, considerably.
Second, theoretically, the best results can be obtained using a laser beam with an optimum spot size for each scan resolution in combination with a graphic arts film or plate characterized by a high gradient and steep toe. This requires imagesetters with tight manufacturing tolerances and films with special (so-called "explosive") development techniques and reduces the working latitude considerably.
Before describing a third approach in solving the aforementioned problems by means of a so-called euclidean dotshape, it may be remarked that theoretically a screen can be designed to produce virtually any shape of dots, but in practice generally square shapes and elliptic shapes are used. Reference can be made to FIG. 9.1 which shows a halftone screen comprising square dot shapes, and to FIG. 9.2 which shows a halftone screen comprising elliptical dot shapes.
As just initiated, a third approach in solving the aforementioned problems uses a so-called euclidean dotshape and is commercially applicated in filmrecorders of the Selectset and Accuset series of the company Miles Inc. Agfa division. This approach, called "Agfa Balanced Screening"--shortname "ABS", tradename .TM.--, is protected by e.g. EP 0 454 274 A2 (in the name of AGFA Corporation), describing a method for controlling halftone dot shape during dot growth. Herein the dot shape gradually changes during dot growth from 0% to 100%, for both standard quadratic screens (cfr. FIG. 9.1) and for elliptical screens (cfr. FIG. 9.2). More specifically the shape of the halftone dot during dot growth changes from circular at the origin of 0% to square at 50% and back to circular at 100%. Thus, the dots grow through a shape sequence of round at the beginning, through rounded square to square at 50%, and the back to rounded square to finally round again at 100%.
Yet, even in screening systems with an euclidean dotshape, some deficiencies in recording small halftone dots still remain perceptable: no consistent reproduction is possible in the extreme ranges of the tone scale (say from 0 to 3% or from 97 to 100%).
From the previous explanation follows that a need exists for a halftoning system that provides a consistent reproduction of the halftone dots across the full tone scale.